The present invention relates to devices for measuring direct electrical current; and more particularly to such sensors which are isolated from the components which conduct the direct current being measured.
Current sensors find common application in many electrical power systems such as automotive load monitoring and control. Applications of such sensors include electrical overload detection, battery condition monitoring and excessive battery discharge warnings. In such applications, the currents can range from less than one ampere to tens of amperes. In non-automotive applications, it is desirable to measure direct currents of 100 amperes or more.
A common technique for sensing direct current was to provide a shunt path in which a known fraction of the total current flows and incorporate a current sensing device in the shunt path. By measuring the amount of current in the shunt path and knowing the fraction of the total current which flowed through the shunt, the level of the total current flowing in the circuit was determined. The disadvantage of shunt type sensors is that the sensor must be physically connected to the current conductor, often adding additional load to that circuit.
Another type of current sensor utilized a transformer which isolated the sensor from the circuit conducting the direct current being measured. In this case, the unknown direct current flowed through a conductor that passed directly through the center opening of a toroidal transformer thereby forming a single turn primary winding of the transformer. A secondary winding of the transformer had 1,000 to 5,000 turns. The flow of direct current through the primary conductor saturated the core of the transformer.
To sense the direct current, pulses of current were applied to the secondary winding in a direction to counter the saturation of the transformer core. Specifically, the current pulse in the secondary winding produced magnetic flux in the winding which countered magnetic flux produced by the direct current through the primary winding. When the two magnetic fluxes were equal, the magnitude of the current in the secondary winding stopped increasing. By sensing the peak current through the secondary winding and knowing the turns ratio between the primary and secondary windings, the magnitude of the direct current in the primary winding can be calculated. A current sensor of this type is described by Rudolf Severns in an article entitled, "Improving and Simplifying HF DC Current Sensors," published in 1986 by the Institute of Electrical and Electronic Engineers.
A transformer isolated direct current sensor can be modified to sense current conducting in either direction through the primary circuit. In this case, two transformers are used with each one being wound so that one will saturate when current flows in one direction through the primary circuit and the other transformer will saturate when the current flows in the opposite direction. Although such sensors measured the magnitude of the current flowing in either direction, they did not produce an indication of the polarity of that current.
Another problem was that conventional transformer isolated direct current sensors were unable to measure very small current levels. Even using transformer cores of high magnetic permeability, such current sensors commonly could not accurately measure currents below 0.1 ampere. Thus, a sensor of this type could not be used in circuits with relatively small current levels.